In the past, numerous methods have been proposed for treating an exhaust gas containing volatile hydrocarbons. In particular, combustion, adsorption, absorption, membrane methods, and the like are known as treatment means for preventing pollution and satisfying legally mandated limits for the concentration of volatile hydrocarbons in exhaust.
Among these, combustion methods are advantageous in that the methods can be regarded as the only methods that can achieve zero emissions when treating a large quantity of exhaust gas in which the volatile hydrocarbons contained in the gas are extremely diluted, i.e., a large quantity of exhaust gas in which the content of volatile hydrocarbons is on the order of a few dozen to a few hundred parts per million, and the quantity of the gas is several thousand to several tens of thousand cubic meters per hour. Furthermore, both regenerative combustion methods and autothermal catalytic combustion methods are simpler than an adsorption method, absorption method, membrane method, or other methods, and can be readily carried out extremely inexpensively.
However, the overriding drawback of such combustion methods is that not only is it impossible to reuse the volatile hydrocarbons, a “massive release of carbon dioxide gas” accompanies the combustion. In this respect, exhaust regulations for carbon dioxide gas, which is the primary cause of global warming, have recently been legally mandated by the “Kyoto Protocol,” and the combustion method may soon be under debate in consideration of the carbon-emissions tax that may be enacted in the near future.
On the other hand, the adsorption methods, which are widely used as an alternative to the combustion methods, surpass the combustion methods when the concentration of volatile hydrocarbons contained in the exhaust gas is on the order of several percent and the amount of exhaust gas to be treated is several thousand cubic meters per hour or less. Specifically, these methods are advantageous in that the treatment unit is compact, and large quantities of volatile hydrocarbons can be directly recovered and reused without burning.
However, the following problems arise when treating large quantities of exhaust gas containing extremely dilute volatile hydrocarbons. Namely, when a conventional adsorbent is used; e.g., a hydrophobic silica gel, zeolite, or a microporous activated carbon, the time necessary for adsorption, i.e., the contact time with the adsorbent, is determined in advance according to the adsorbent that is used. For example, the contact time for activated carbon is about 2 to 5 seconds, and the contact time for hydrophobic silica gel is 10 to 15 seconds. When the contact time exceeds these standards, the volatile hydrocarbons in the exhaust gas pass through the adsorbent layer before being adsorbed by the adsorbent. Consequently, when a large quantity of exhaust gas is treated, unnecessarily large quantities of adsorbent must be used in order to achieve a contact time necessary for adsorption.
Furthermore, the abovementioned adsorbents, and particularly zeolite or microporous activated carbon, have excellent adsorption performance, but poor desorption performance. The majority of these adsorbents are therefore desorbed as a result of heat desorption caused by steam or by air heated to a high temperature (see Non-Patent Reference 1).
However, the reason why these poorly desorbing adsorbents are still widely used at present is that causing the exhaust gas to pass through the adsorbent, brings the concentration of the dilute hydrocarbons to 5 to 10 times the concentration obtained using the combustion method, in which the dilute volatile hydrocarbons are combusted.
An advantage is accordingly realized in that the hydrocarbons dissolved in water can just barely be recovered even when using steam desorption.
A “fibrous activated carbon system” and a “honeycomb rotor system” are typical examples of systems using these adsorbents.
The “honeycomb system” is advantageous in that dilute hydrocarbons can be increased to a self-combustible concentration by desorption using high temperature air. In this system, which uses a honeycomb-shaped (hexagonal) rotor on which activated carbon or zeolite is specially formed, large quantities of exhaust gas containing dilute volatile hydrocarbons are passed through the rotor, and only the volatile hydrocarbons are adsorbed thereon. The adsorbed portion is transferred to the desorbed portion by the rotation of the rotor, desorbed by air that has been heated to approximately 200° C., and extracted from the system at a self-combustible concentration; i.e., as a concentrated gas having a concentration of approximately 2000 to 3000 ppm. This series of steps can be continually carried out by the rotational movement of the honeycomb rotor.
The “fibrous activated carbon system” and the “honeycomb rotor system” are both well-known in the prior art. However, problems have arisen in that the concentration of the concentrated volatile hydrocarbons is at most 1000 to 5000 ppm, and it is less economical to extract volatile hydrocarbons of this concentration for recovery purposes without causing them to combust. The present inventors have therefore experimented with a novel approach for solving these problems
Specifically, 99% or more of the volatile hydrocarbons contained in the exhaust gas can be recovered by changing the adsorbent used in the systems described above to “an agent that can be desorbed merely by the combined use of a vacuum and/or room-temperature nitrogen and ambient air,” or by using an adsorption/desorption unit that has been previously developed and proposed by the present inventors (see Patent Reference 1 described below) to treat the thus-extracted volatile hydrocarbons, which have been concentrated to a certain degree.
This agent; i.e., adsorbent, is a novel activated carbon that can be desorbed merely by vacuuming and purging means, rather than heat desorption using steam. For example, an activated carbon using a raw material such as brown coal or peat as a base has been developed in China; an activated carbon having olive pits as a raw material has been developed in the Netherlands; and an activated carbon having specific wood materials as a raw material has been developed in the US. All of these activated carbons have a bore diameter of 10 to 60 Å (1 to 6 nanometers). In automotive vehicles in Europe, the US, and other developed nations other than Japan, obligatorily registered “canisters” comprise such activated carbons. Gasoline vapor that leaks out during travel is adsorbed into these canisters, and, once a certain amount of vapor has accumulated, is desorbed upon fresh air being drawn in from the exterior. The purged exhaust gas is caused to combust in the engine compartment. It is entirely unnecessary for heating to be performed when desorption is performed.
Prior to the present invention, the present inventors had proposed “a method for treating an exhaust gas containing volatile hydrocarbons” using the activated carbon independently, or by combining the activated carbon with synthetic zeolite or hydrophobic silica gel, which are widely used in the prior art, to form multiple layers (see Patent Reference 1).
This treatment method is characterized in being “a method involving the use of an adsorption layer loaded with a mesoporous activated carbon precoated with volatile hydrocarbons, or a multilayered loaded layer in which a similarly precoated hydrophobic silica gel and/or zeolite are combined; during desorption, a purge gas is used in conjunction with a vacuum pump, and the adsorption/desorption switching time is 1 to 30 minutes.”
Examples of methods that are related to an adsorption separation technique and that are generally used in the field include “a method involving the use of an activated carbon as an adsorbent; and, during desorption, solely using steam or another similar heating medium to carry out desorption (see Non-Patent Reference 1 cited above),” and “a method involving the use of a synthetic zeolite precoated with gaseous hydrocarbons, hydrophobic silica gel, or another non-combustible adsorbent; and alternating between an adsorption operation and a desorption operation (see Patent Reference 2).”
Housing a cooling coil inside an adsorption column has traditionally been a commonsensical approach for “preventing a massive build-up of heat due to adsorption heat.” A method has also been proposed in which volatile hydrocarbons (in liquid form) obtained by cooling a purge exhaust gas are circulated in the adsorption column instead of cooling water (see Patent Reference 3).
[Non-Patent Reference]: “Separation Technology,” Vol. 33, No. 4 (of 174 consecutive volumes); published by the Society of Separation Process Engineers; Jul. 31, 2003; pp. 18 to 20.
[Patent Reference 1]: Japanese Laid-open Patent Application No. 2004-42013 (Claim 1)
[Patent Reference 2]: Japanese Laid-open Patent Application No. 09-057060 (Claim 1)
[Patent Reference 3]: Japanese Examined Patent Application No. 59-50715 (Claim 1)